Build policy on top of mechanism.

• Usually, the same functionality is required under different policies.
• Enhances reusability of lowest-level components.
• Use contextual parameterization to generate components operating under various policies.
• Use mixin inheritance to construct subclasses sharing a policy attribute.
• Example: Layer a component that checks preconditions on top of one that doesn't.
• Example: Generate mediators that rely on the same underlying components but under different protocols (e.g., point-to-point vs multicast, synch vs asynch communication).